As a self-luminous display device, recently, attention has been given to an electroluminescence (EL) display device using an EL element. In addition, there has also been developed an active matrix-type EL display device in which pixel circuits each including an EL element and a thin-film transistor (TFT) are arranged two-dimensionally.
An organic EL element, which is one example of EL elements, has a sandwich structure in which a light emitting layer made of an organic substance is interposed between an anode and a cathode. In the organic EL element, when a voltage is applied between the anode and the cathode, holes are injected from the anode into the light emitting layer while electrons are injected from the cathode into the light emitting layer. The hole and the electron each injected are recombined with each other in the light emitting layer to generate an exciton, and the generated exciton emits light upon annihilation. Thus, the organic EL element emits light.
In an active matrix-type EL display device, an EL element emits light at a brightness according to a drive current supplied from a drive TFT in a pixel circuit that includes the EL element. One of two electrodes between which a light emitting layer is interposed is made of a transparent material; therefore, the light emitted from the light emitting layer transmits through the transparent electrode and, then, goes out of the EL display device. Thus, the active matrix-type EL display device performs surface light emission to display a screen.
In the active matrix-type EL display device, an electric current must be supplied to the pixel circuit through a power source line in order to allow the EL element to emit light. For example, a pixel circuit (a typical pixel circuit having a configuration called 2 TFT+1 C) shown in FIG. 2 performs a constant current drive operation of supplying a signal potential at a data line 2 to a gate terminal of a second TFT 32 (which functions as a drive TFT) through a first TFT 31 (which functions as a switch) and, then, supplying an electric current according to a conductance of the second TFT 32 to an EL element 10. In general, however, when an electric current flows through a line having a resistive component, a voltage drop occurs. In the active matrix-type EL display device, consequently, there is a difference in power source voltage to be supplied between a pixel circuit arranged at an end portion and a pixel circuit arranged at a center portion, so that unevenness in brightness occurs at a display screen in some instances.
Specifically, an electric current Id which flows through a drive TFT is given by the following expression (1) in consideration of a channel length modulation effect.Id=(½)μCox(W/L)(Vgs−Vth)2(1−λVds)  (1)
In the expression (1), μ represents a mobility, Cox represents a capacity of a gate oxide film, W/L represents an aspect ratio, Vgs represents a gate-to-source voltage, Vth represents a threshold voltage, λ represents a channel length modulation coefficient, and Vds represents a drain-to-source voltage.
A source potential varies by a voltage drop occurring at a power source line, so that the gate-to-source voltage Vgs and the drain-to-source voltage Vds change. As shown in the expression (1), then, the electric current Id supplied from the drive TFT to the EL element changes, so that the brightness of the EL element also changes. Consequently, the unevenness in brightness occurs at the display screen.
In order to prevent the unevenness in brightness, preferably, a resistance of the power source line is decreased and a potential at the power source line is made constant. The low resistance of the power source line allows not only prevention of the unevenness in brightness, but also reduction in Joule heat generated at the power source line and reduction in electric power consumed by the power source line.
As a method of decreasing the resistance of the power source line, there have been considered a method of making a width of a power source line wide and a method of making a thickness of a wiring layer large. As shown in FIG. 11, moreover, Patent document 1 discloses a method of providing power source lines VL arranged in parallel with data lines DL and, additionally, bypass power source lines BL arranged in parallel with scanning lines GL and establishing electric connection between the power source line VL and the bypass power source line BL through a contact C1, thereby providing the power source lines in a lattice shape. As shown in FIG. 12, Patent document 2 discloses a method of establishing electric connection between a power source line VL and a light shielding film BM (a portion surrounded by a bold line) through a contact C2, thereby connecting the light shielding film to the power source line in parallel.
Patent document 1: JP 2001-100654 A
Patent document 2: JP 2001-100655 A